The invention relates to a method for cleaning the spray electrode of an electrofilter.
A conventional is shown and described in method EP 0 433 152 A1. In the known method, the filtration performance of the electrofilter is adversely affected to a considerable degree while the spray electrode is being cleaned with a cleaning body. The cleaning body can be passed over almost the entire length of the spray electrode. To eliminate operating problems, it is therefore suggested to operate several electrofilters simultaneously and to clean the spray electrode of only one of the several electrofilters at a time. In this manner, the total filtration performance is adversely affected to a comparatively slight extent by the cleaning. The considerable space requirements and the higher manufacturing costs of such an arrangement of a plurality of electrofilters is taken into account.
A conventional electrofilter is also shown and described in this same publication in which the above-mentioned problems occur. The spray electrode in the form of a wire has a comparatively long length. It is therefore sensitive to vibrations. This adversely affects the choice of possible areas of application. The conventional electrofilter is configured to remove dust from gases.
An object of the invention is to provide an electrofilter that is sturdy, economical to manufacture, and permits a high constant filtration performance and to provide a method that ensures reliable cleaning of the spray electrode without adversely affecting the filtration performance.
Therefore, according to the invention, instead of the vibration-sensitive wire part, a two-stage design is provided for the spray electrode. A first stage has a comparatively small diameter and a free end. The corona is formed on this first stage, especially at the free end. This first stage can be made comparatively short. The second stage, with a larger diameter and longer by comparison, serves only to maintain the electrical field so that the initially ionized particles can be deposited reliably on the precipitation electrode.
Regular cleaning is required especially in the corona zone of the spray electrode since firmly adhering deposits form there over time even if, theoretically, solids are not to be filtered out aerosols containing oil, such as for example the vent gases from the crankcase in an internal combustion engine are to be filtered.
The two-stage design of the spray electrode makes the latter not only sturdy and insensitive to vibrations but also, because of the different density of the field lines, the solids are deposited almost exclusively on the first stage. Cleaning can therefore be limited to this area with a comparatively short length. Therefore, a correspondingly short-stroke drive for the cleaning body is sufficient that can be accomplished by simple design and economical means.
In addition, the energy to guide the cleaning body on the spray electrode can be provided advantageously exclusively by energy from the engine so that additional driving elements, in the form of an electric drive for example, can be eliminated which are expensive and can be troublesome because of the heat and vibration effects.
For example, an expansion body filled with fluid or gas can be provided connected thermally with the engine and heated by the operation of the engine; the cooling of the engine while it is at rest causes a backwardly directed movement of the expansion body and the cleaning body associated therewith, with the spray electrode being cleaned during this movement.
In addition, pressures or vacuums developed by the engine, in gases or oil for example, can be used to move a membrane that moves the cleaning body into a starting position so that the rearwardly directed movement of the cleaning body takes place during the subsequent shutdown of the engine when the pressure or vacuum is no longer maintained.
This backward movement can be effected by the reduction in the volume of the expansion fluid or by the spring force of the membrane or an additional spring, with the cleaning body being held against the action of the spring during engine operation in a position in which it does not abut the spray electrode so that optimum precipitation performance of the electrofilter is ensured when the engine is running. Alternatively, provision can be made to design the cleaning body and the movable parts connected with it as a spring-mass system so that with certain vibrations of the engine a resonant frequency of this spring-mass system is reached that causes the cleaning body to vibrate so that the body performs its cleaning movement along the first stage of the spray electrode.
Cleaning of the first stage can be made especially simple and functionally reliable if its cross-section remains constant over its length and permits a uniform application of the cleaning body during its movement. For this purpose, this first stage advantageously has a constant cross sectional contour so that a good fit between the cleaning body and the first stage can be ensured. Depending on the selected manufacturing method for the spray electrode, a cross-sectional constant that is not completely identical is reached over the entire length of the first stage. Thus, for example, when casting the electrodes, a certain taper may be necessary to facilitate the removal of the cast electrode body from the casting mode.
In other words, the invention proposes regular cleaning without costly sensory mechanisms or an additional time-measuring device in which the cleaning body is moved along the spray electrode, always at certain operating states of the engine. For example, such a cleaning cycle can be triggered with the engine at rest. Even with relatively long operating times which can occur for example in commercial vehicles such as trucks, buses, or taxis, regular sufficiently frequent cleaning of the spray electrode can be ensured in this manner to guarantee constantly good filtration properties of the electrofilter.
The constantly high filtration performance is achieved with this regular cleaning and can also be supported by the fact that during engine operation the cleaning body is basically not moved along the spray electrode and so the performance of the spray electrode is not adversely affected.
Thus, provision can be made to move the cleaning body by engine power into a starting or resting position only when engine operation begins, in which position it is at a distance from the tip of the spray electrode that forms the corona and from which it starts the cleaning of the spray electrode when the engine is at rest.
However, even if cleaningxe2x80x94as a function of vibration for examplexe2x80x94takes place during engine operation, adverse effects of filter performance are comparatively slight since, because of the short length of the first stage of the spray electrode, the distance traveled by the cleaning body is very short and cleaning takes place in a correspondingly short time. It is therefore not necessary to provide additional electrofilters that must be cleaned alternately and to take the associated disadvantages into account.
Embodiments of the invention will be explained in greater detail below with reference to the drawing.